The present invention relates to a method and an apparatus for controlling the combustion of gasified fuel in a system comprising the steps of gasifying a low quality fuel such as coal or heavy oil, combusting the gasified fuel and utilizing the combustion gas for generating power.
An example of the system for combusting the gasified fuel to generate power is a coal-gasification power plant based on the prior art of oxidizing coal with air, and utilizing the resulting gasified fuel as a gas turbine fuel, on which the following description will be based.
As shown in FIG. 1, the coal-gasification power plant comprises a combustion unit, a gas turbine unit, a generator unit and a coal gasification unit, and the combustion unit comprises a compressor 1 and a combustor 2. Air 4 is compressed by compressor 1, and the compressed air 5 is led to combustor 2. Coal-gasified fuel 18 generated in the coal gasification unit is also led to combustor 2 and combusted therein with air 5 as an oxidizing agent to produce combustion gas 7 at a high temperature and a high pressure.
Combustion gas 7 is led to turbine 3 to drive it. Most of power generated by turbine 3 is consumed for revolution in generator 8 to generate electric power. A portion of the power generated by turbine 3 is consumed for revolution in compressor 1.
Flue gas 9 from turbine 3 is led to a waste heat recovery boiler 10 to generate steam at a high temperature and a high pressure, and the generated steam is consumed for driving a steam turbine (not shown in the drawing) and then for revolution in a power generator (not shown in the drawing) connected to the steam turbine to generate electric power.
The coal gasification power plant uses a gasified fuel produced in the coal gasification unit as a fuel in place of the conventional fuel of high heating value such as natural gas, light fuel oil or kerosence which will be hereinafter referred to as "natural gas, etc.".
Generally, coal gasification is based on thermal decomposition of coal at an elevated temperature to gasify it, and the example shown in FIG. 1 is based on combustion of a portion of coal with air and thermal decomposition of the remaining portion of coal by the resulting heat of combustion, which is called "gasification by partial oxidation".
The coal gasification unit based on such partial oxidation of coal, shown in FIG. 1 comprises a coal gasifier 11, a heat recovery boiler 12 and a gas purification device or apparatus 13.
Raw material coal 14, air 15 as an oxidizing agent, and steam 16 for adjusting the gasification temperature and also enriching hydrogen in the gasified fuel by steam reforming are led to gasifier 11, where a portion of coal 14 is combusted with air 15 and the remaining portion of coal 14 is thermally decomposed by the resulting heat of combustion with temperature adjustment of steam 16, whereby gasified fuel 17 is generated as a coal gas containing hydrogen. Air 15 is a portion of air 5 extracted from the line from compressor 1 to combustor 2, and the extracted air is compressed by a boost-up compressor 6 and led to a gasifier 11. In place of the extracted air 15, an oxygen-enriched gas from a free standing oxygen plant can be supplied thereto.
The raw gas 17 generated in coal gasifier furnace 11 is led to heat recovery boiler 12, and heat-exchanged with a heat-exchanging medium therein to recover the sensible heat possessed by raw gas 17, for example, by generating steam.
After the heat recovery, raw gas 17' is led to gas purification apparatus 13, where dusts contained in the gas, reaction products containing sulfur, nitrogen, etc. originating from coal such as H.sub.2 S and NH.sub.3, and alkali metals such as Na, K, Ca, etc. generated in a vapor state are removed from the gas, whereby purified gas 18 is obtained. The purified gasified fuel 18 is led to combustor 2, as described heretofore.
The coal gasification power plant includes a fuel production unit and is characterized by power generation while producing the necessary fuel, as compared with the one using fuel of natural gas, etc.
The properties and characteristics of coal-gasified fuel for the coal gasification power plant are distinguished in the following points from those of natural gas, etc.
(1) Heating value of coal-gasified fuel is 1/7 to 1/10 of that of natural gas, etc., because air as an oxidizing agent for coal and steam are used, so that inert gas such as N.sub.2, CO.sub.2, H.sub.2 O, etc. are contained in the gasified fuel, and a portion of the coal is combusted at the gasification of coal to thermally decompose the remaining portion of coal by the resulting heat of combustion. The heating value of coal-gasified fuel is as low as 1,000-1,500 Kcal/Nm.sup.3. In other words, it is necessary to increase the flow rate of gasified fuel to a combustor to produce a combustion gas at the same temperature as that obtained by combustion of natural gas, etc., because combustion is usually carried out in excess of air in the case of a gas turbine. Otherwise, combustion becomes unstable in the combustor.
The minimum necessary heating value of low calorific fuel for maintaining stable combustion depends upon composition of fuel, particularly hydrogen content, structure of combustor and air-fuel ratio, and is 800-1,000 Kcal/Nm.sup.3 according to the so far available results of relevant tests. It is known that combustion becomes unstable, or combustion is impossible to continue by blow-out, etc. below the minimum necessary heating value. Particularly, in the case of low load driving of a gas turbine, the air-fuel ratio becomes higher and fuel must be combusted in a lean state.
(2) Heating value, flow rate and temperature of coal-gasified fuel depends upon change in composition of coal, fluctuation in the load of coal gasifier change in the operating conditions of gas purification apparatus.
In contrast to the case of using natural gas, etc. as fuel, the coal gasification unit must be operated at the same time when the gas turbine is to be driven. The heating value, temperature and flow rate of coal-gasified fuel change during the operation by change in operating conditions of the coal gasification unit or uneveness in quality of coal as a raw material.
Combustion gas temperature T.sub.g as a combustor outlet temperature in a gas turbine unit can be represented as below in a form of function by the following factors: ##EQU1## wherein C.sub.a, C.sub.f and C.sub.g represent specific heats at constant pressure of air, fuel and combustion gas, respectively, G.sub.ao and G.sub.f represent flow rates of air at the compressor outlet and fuel, respectively; Ta, Tf, and T.sub.g represent temperatures of air, fuel and combustion gas, respectively; H.sub.u represents heating value of fuel; and k is a constant, which can be represented by the following equation, if the flow rate of extracted air from the air to be led to the coal gasifier by the compressor is G.sub.EX and if fuel flow rate G.sub.f is proportional to G.sub.EX : EQU G.sub.EX =kG.sub.f ( 2)
In the case of natural gas, etc. or gasified fuel obtained by gasification of low quality fuel with oxygen, k=0 in the above equation (2).
Since in the case of natural gas, etc. specific heats at constant pressure, C.sub.a, C.sub.f and C.sub.g, temperatures of air and fuel, T.sub.a and T.sub.f, and heating value H.sub.u are substantially constant, the combustion gas temperature T.sub.g is dependent only on the fuel flow rate G.sub.f. Thus, adjustment of combustion gas temperature T.sub.g, that is, load adjustment of gas turbine, can be made only by controlling the fuel flow rate G.sub.f, and very stable control can be carried out.
On the other hand, in the case of gasified fuel as in the present invention, heating value H.sub.u and fuel temperature T.sub.f fluctuate as described above, and thus combusting gas temperature T.sub.g is a function of three variables, i.e. fuel temperature T.sub.f, heating value H.sub.u, and fuel flow rate G.sub.f, and cannot be effectively controlled only by simple detection and control of fuel flow rate G.sub.f as in the case of natural gas, etc. Particularly in the case of coal-gasified fuel, a higher fuel flow rate G.sub.f is required than that in the case of natural gas, etc., and thus fluctuation in fuel temperature T.sub.f, i.e. fluctuation in the sensible heat of fuel to be led to the combustor, gives a great influence upon the combustion gas temperature T.sub.g. To test a combustibility of a coal-gasified fuel, a combustion test for analyzing a combustion gas by sampling and combusting a portion of gasified fuel has been proposed (FE-1806-22 ERDA (1977) pages 6-33, 6-34, FIG. 6.3.8).